The present invention relates generally to the control of DC motors, and particularly, but not exclusively, to a method and apparatus providing electronic rotor pointing with high angular resolution using a sensorless permanent magnet DC motor.
There are numerous techniques in the prior art for controlling the positioning of a rotor of a permanent magnet (PM) DC motor. These techniques can be generalized into two major categories. The first category generally includes those techniques in which a PM DC motor is designed to move at a precise speed in synchronism with, or locked to, the waveform of the driving voltage or current which energizes the windings. PM stepper motors, which have a permanent magnet in the form of a rotor magnetized in alternate polarity “stripes” parallel to the rotor shaft, are used with this type of technique. The step size (angular resolution) of such a motor is entirely a function of the angular “width” of these magnetized stripes, and an angular resolution of 7.5 degrees is common in most inexpensive motors. However, costs increase greatly if such motors are to provide high resolution in the 2 to 5 arc minute range. Additionally, PM stepper motors move in steps by sending pulse trains of varying polarity to multiple windings. The frequency of the pulses and the phasing between the pulses applied to the various windings determines the speed and direction of motor motion, respectively. As a result, precise control over rotor positioning is provided, but at the cost of control circuitry complexity.
The second category of PM DC motor rotor positioning techniques includes those in which sensors are external to, or built into, the motor. Typically, such position sensors include Hall effect sensors and optical encoders. With Hall effect sensors, resolution is limited by the number, the positioning accuracy, and the gain tolerance of the sensors. With optical encoders, high resolution is provide at a higher cost. Optical encoders require electronics for decoding and accumulation. Such a system requires initialization, and over time may also require alignment and adjustment. Accommodating either Hall effect sensors or optical encoders also increases the size of the resulting device.
Conventional laser levels typically use the rotor of a sensorless permanent magnet DC motor to rotate a prism. The prism reflects a beam of laser light used in leveling operations. The operator controls such a sensorless DC motor using open loop control (no feedback) and therefore, moves the beam at a desired speed by increasing or decreasing the DC voltage applied to the motor. In order to position a beam in a desired direction with such an arrangement, the operator typically jogs the DC motor (applies a succession of voltage pulses) to point the laser beam at a small, distant object. However, with open loop control, rotor pointing by jogging the motor is inaccurate, often unrepeatable, and can be frustrating to the operator due to the lack of reliable beam pointing.
For example, stopping the laser beam on a two-inch wide object 100 feet away requires a DC motor speed of about one revolution per minute given an average human reaction time of 100 ms and a typical 6:1 drive ratio. However, rotating a DC motor at such slow speeds is problematic, since a motor's resolution changes with temperature and at different angular positions due to unavoidable variations in the manufacturing process and wear patterns of a motor's bearings. These variations make fine positioning operations at such a slow speed difficult with open loop control of a PM DC motor. However, due to space and cost considerations, using a stepper motor and/or position sensors in a laser level for closed loop rotor positioning control of the DC motor to provide acceptable resolution is not economical.
Therefore, a method and apparatus are needed for providing electronic rotor pointing to a sensorless permanent magnet DC motor, which take into account friction, temperature, bearing manufacturing, and wear pattern variations to provide accurate, repeatable, and reliable resolution position control.